1. Field of the Invention
The present invention relates to gelled fracturing fluids of the type used in well bore operations, to an improved polymer for use in such fluids, and to a method of treating or fracturing a subterranean formation using such a fluid.
2. Description of the Prior Art
Hydraulic fracturing is a process to enhance the production of oil and gas from wells drilled through hydrocarbon bearing subterranean formations. It can also be used to improve the water injection characteristics of water injection wells used to support hydrocarbon production in water flood projects. Most often, the process occurs by injecting a viscous fluid through the well tubulars at suitable rates and pressure, allowing the formation in contact with the tubulars to respond by fracturing. Once the fracture in initiated and grows at a steady rate, a propping agent is added to the viscous gel and is pumped into the growing fracture. Once all the proppant-laden fluid is pumped, a final proppant free fluid is pumped as a flush to clean the tubulars of proppant-laden fluid.
Afterwards, the gel slowly degrades to low viscosity and the fracture relaxes and closes on the proppant pack. When the well is opened and the thin treating fluid flows out of the well, the well is said to be cleaning up. The degree of clean-up most often determines ultimate hydrocarbon production from the well. One factor that hinders clean-up is the amount of gel residue left in the proppant pack from the viscosifying agent. The preferred viscosifying agents are polysaccharides (galactomannans) such as guar gum and guar gum derivatives such as carboxymethyl guar, carboxymethylhydroxypropyl guar or hydroxypropyl guar, and to a much lesser extent, cellulose derivatives such as hydroxyethyl cellulose or carboxymethylhydroxyethyl cellulose. One of the most preferred viscosifying agents is guar gum. Most often, the polymer is used from 0.24 to 0.72% (wt/vol). The viscosity of the polymer solution is enhanced by crosslinking the polymer to form a gel. One preferred method to gel the polymer solution is to crosslink the fluid with borate compounds in alkaline environments. References to this technology include that found in U.S. Pat. No. 5,160,643 (Dawson); U.S. Pat. No. 5,145,590 (Dawson); U.S. Pat. No. 4,974,077 (Free) and U.S. Pat. No. 4,619,776 (Mondshine). The pH is often adjusted to alkaline values with buffers such as potassium carbonate or mixtures of potassium carbonate and potassium hydroxide. The optimum pH ranges from about 9.5 to 11.5. The most preferred ranges are from 9.8 to 10.5. The borate crosslinker can be any borate ion source and includes boric acid, sodium borate, including anhydrous or any hydrate, borate ores such as colemanite or ulexite and any borate complexed to organic compounds to delay the release of the borate ion such as is taught in U.S. Pat. No. 5,145,590. Because various sources of borate ions each have their own preferred concentration based on the particular job application pH and temperature, the optimum borate ion concentration is that which provides the best gel under the given conditions. This usually ranges from 0.001% to 1.5% (by wt of aqueous fluid) of crosslinking agent and the most preferred being 0.005% to 1.0% (by wt of aqueous fluid).
Although guar-borate gels satisfy most properties necessary to successfully fracture formations, they do have some limitations. For example, although such gels can be degraded by enzymes such as the system described in U.S. Pat. No. 5,067,566 (Dawson), and oxidants such as described by U.S. Pat. No. 5,624,886 (Dawson) and U.S. Pat. No. 5,253,711 (Mondshine), substantial formation damage inhibiting production can still exist. In an effort to reduce such damage, another approach, used together with gel degrading substances, is to use less guar gum as described by U.S. Pat. No. 5,681,796 (Nimerick). In the case of the '796 patent, the polymer concentration was reduced to less than 0.24% by using a special alkaline buffer system. The viscosity of a 0.18% (wt/vol) guar gel was shown initially to be 140 cP at 100 sec−1 and 140° F. and after 3 hr was 105 cP. In another example, the viscosity of a 0.24% (wt/vol) guar gel was shown initially to be 285 cP at 100 sec−1 and 175° F. and after 3 hr declined to 180 cP. Although the viscosity shown is greater than that of a polymer solution having a guar polymer concentration ranging from 0.24 to 0.72% (wt/vol) it is lower than most fluids used in hydraulic fracturing applications. In those treatments, the viscosity used to fracture wells is normally two to five times greater than that shown in U.S. Pat. No. 5,681,796. Most often when using less polymer in the gel, the essential properties necessary to perform the fracturing treatment suffer, often limiting the treatment to a small percentage of wells.
Thus, while improvements have been made in the field of galactomannan based fracturing fluids, a need continues to exist to provide fluids with adequate viscosity, fluid loss and other essential properties for performing a fracturing operation, while at the same time being less damaging to the surrounding formation than the prior art treatments.